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Your irrigation system operates by sending out an electrical signal to the valves that make the sprinklers turn on. It's a timer – it turns on the sprinklers every night or every other night, rain or shine. When you install a rain sensor, you're essentially installing an on/off switch for the system. You set the sensor to turn the system off when a certain amount of precipitation has occurred (say, 1/8th of an inch). If you get 1/4 inch of rain tonight, the sensor would flip the switch to the system, so it wouldn't turn on to water your already soggy grass. But if you get a little shower that doesn't even amount to an 1/8th inch of rain, your system will continue watering.
Note: THIS IS NOT AN INSTALLATION DIAGRAM!
How the sensor works: There are a series of discs in the sensor that expand when they get wet. Once the discs get to a certain size, they flip a microswitch, which interrupts the current going from the controller to the valves. So when the controller tells the valves to start watering in the middle of a rainstorm, the valves don't get the memo – they don't turn on. When the discs dry out in the sun and the wind, the switch flips back, and the valves will water the next time the controller tells them to start watering.
Perhaps you have a light in your living room that's on a timer, but it's plugged into an outlet that is operated by a light switch. Now, as long as you leave that light switch on, the light is going to turn on and off at the same times every single night. But if you flip the light switch, when the usual time comes around for the light to turn on, it can't – it doesn't have power. That's all that a rain sensor does. It's a switch activated by the rain. And the clock is sophisticated enough that you don't even have to reset the timer after you've flipped the switch.
What about a moisture sensor? The biggest difference between a rain sensor and a moisture sensor is that the moisture sensor measures the amount of moisture in the ground, instead of simply registering the amount of rain that has fallen. It gives a much more accurate reading of whether the grass needs water because it goes right to the source: the soil that the grass grows in.
We recommend that your rain or moisture sensor be installed by a professional contractor. If you don't know who your irrigation contractor is, chances are that they put a sticker on your controller. Stroll out to the controller and have a look. They'll be able to tell you whether you have a rain sensor, and how much one would cost to install. If you can't find any information on your irrigation contractor, click here to get a referral.
Think you'd like to try to install one yourself? Click on the appropriate link for more information: Rain Sensor Instructions | Wireless Rain Sensor Instructions
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